1. Field of Invention
This invention pertains to a novel apparatus and process for controlling etching of silicon-based or organic materials on large rectangular substrates for manufacture of flat panel displays or other devices.
2. Related Art
There are many significant technical problems with scaling well-known methods for dry etching and photoresist ashing to the very large substrates such as fifth generation TFT/LCD (thin film transistor liquid crystal display) flat panel substrates or large solar panels. The best-known and least expensive approach for etching is with parallel plate, RF discharges such as have been used for decades in manufacturing integrated circuits. For ashing in IC manufacturing the proven parallel plate resist stripping method found in U.S. Pat. No. 5,189,634 was able to achieve good results and productivity for 200 mm wafers. However, with increasing substrate size and a rectangular shape, it is increasingly difficult to achieve the uniformity of rate needed for good device yields.
In addition, RF plasma discharge processes are likely to cause substantial electrical potential variations across the surface of the larger substrates. Such potential differences typically increase as the size of the substrate increases and also worsen as non-uniformities in the process increase. Such potential variations can cause electrical stress damage to components being fabricated on the substrate.
Also, the etching rate may have a strong dependency on the gap between the electrodes. Therefore, any variation in the spacing between the showerhead and pedestal is likely to cause non-uniformity in the etching rate, and can also cause substrate-charging problems as well. With large showerheads or pedestals, maintaining a high degree of flatness after processing large numbers of substrates may be very challenging, and the problem is compounded because the relationship between electrode gap and etch rate is non-linear. Areas with a smaller gap have lower impedance to RF current flow, and the increased current flow results in increased power deposition and increased conductivity in such areas. That, in turn, causes even further increase in RF current density in the areas.
Heretofore, etching rate variations and, to a lesser extent, surface potential variations have been minimized by varying the mixture of gases, pressure, RF power, and the spacing between the showerhead and substrate supporting pedestal. This approach has provided adequate uniformity for many generations of integrated circuit fabrication technology on silicon wafers with wafer diameters from 3″ to 12″. However, good uniformity of etching rate has become increasingly difficult as the substrate or wafer size has increased. Getting good uniformity of surface potential has been very difficult even for round wafers of limited size, and remains a significant challenge with this type of etching technology. For the larger and larger rectangular substrates used for making TFT/LCD screens, it has not been possible to achieve satisfactory uniformity of etch rate by scaling up the traditional etching processes.
U.S. Pat. No. 5,532,190 shows a showerhead in which the reservoir is divided into concentric segments in order to achieve a more uniform etch rate. That approach is pretty much limited to round wafers, and it does not address the issue of uniform substrate surface potential which is very important for achieving good yields in devices having thin, sensitive dielectric layers. The concentric segmentation is better suited for round substrates with a symmetric radial flow pattern than it is for rectangular substrates where flow patterns near the corners are more complex and not simply outward from the center of the substrate. Gas flow is not constant in direction as it moves toward corners of a rectangular substrate, and that causes total gas flow to the corners to be reduced. Hence, concentric segmentation would not be very effective with a rectangular substrate.
Applying feedback control of reactant gas flow as suggested by U.S. Pat. No. 5,853,484 to a rectangularly segmented showerhead would require a minimum of 9 segments to distinguish between center region and corners. That would require 9 separate monitoring devices for determining the rates of etching in the 9 different segmented areas of the substrate and 9 sets of gas flow controllers for the gases to be supplied through the showerhead to the plasma for the different segments. Furthermore, increasing the flow for any of the segments would affect the total flow pattern between the electrodes and thereby influence the etching rates on all of the other segments. That means that a very complex, iterative adjustment scheme would be needed to actually accomplish the adjustment of the rates desired. Use of such a rate adjustment method and apparatus would increase cost and complexity of the device and still not be likely to be successful due to the variation of the rate across the area of each of the segments. To be successful, such an approach would require a very large number of segments, possibly 25 or more, and would be prohibitively expensive.